1. Field of the Invention
The present invention relates to an optical recording medium, in particular, a phase-change optical recording medium comprising a recording material capable of causing changes in the phase thereof by the application of a light beam thereto, thereby recording, reproducing and overwriting information therein; and a recording and reproducing method using the above-mentioned phase-change optical recording medium, which is applicable to optical memory devices, particularly, rewritable compact disc (hereinafter referred to as CD-rewritable or CD-RW).
2. Discussion of Background
There is conventionally known a phase-change optical recording medium which utilizes phase changes between a crystalline phase and an amorphous phase or between one crystalline phase and another crystalline phase as one of the optical recording media which are capable of recording, reproducing and erasing information by the application thereto of electro-magnetic waves, such as a laser beam. This kind of phase-change optical recording medium enables the overwriting of information by the application of a single laser beam thereto, although it is difficult to perform such overwriting operation, using magneto-optical memory. An optical system of a drive unit for the phase-change optical recording medium is simpler in mechanism than that for the magneto-optical recording medium, so that research and development of recording media of this kind has been actively conducted in recent years.
As disclosed in U.S. Pat. No. 3,530,441, so-called chalcogen-based alloys, such as Ge--Te, Ge--Te--Sn, Ge--Te--S, Ge--Se--S, Ge--Se--Sb, Ge--As--Se, In--Te, Se--Te and Se--As, are conventionally used as recording materials for the phase-change optical recording media. In addition, it is proposed to add Au to the above-mentioned Ge--Te based alloy to improve the stability of the recording material and also to increase the crystallization rate thereof as disclosed in Japanese Laid-Open Patent Application 61-219692. Furthermore, the addition of Sn and Au to the Ge--Te based alloy, and the addition of Pd to the same are respectively proposed in Japanese Laid-Open Patent Applications 61-270190 and 62-19490 for the same purposes as mentioned above. Furthermore, a recording material comprising a Ge--Te--Se--Sb alloy with a specific composition, and a recording material comprising an Ge--Te--Sb alloy with a specific composition are respectively disclosed in Japanese Laid-Open Patent Applications 62-73438 and 63-228433 for improvement of the recording and erasing repetition properties of a recording medium comprising each recording material.
However, not all the properties required for the phase-change rewritable optical recording medium are satisfied by the above-mentioned conventional phase-change optical recording media. In particular, as the most important targets to be reached, the improvement of recording sensitivity and erasing sensitivity, the prevention of the reduction of erasing ratio at overwriting, and the extension of the life of recorded and unrecorded portions are not yet reached by the above-mentioned conventional phase-change optical recording media.
In Japanese Laid-Open Patent Application 63-251290, there is proposed an optical information recording medium which is provided with a recording layer comprising a single layer of a compound of a multi-component system composed of substantially three or more components in a crystalline state. It is asserted that the single layer of the compound of a multi-component system composed of substantially three or more components contains a compound composed of three or more components with a stoichiometric composition, for example, In.sub.3 SbTe.sub.2, in an amount of 90 atom % or more in the recording layer. It is furthermore asserted that recording and erasing characteristics can be improved by using this kind of recording layer. However, this optical information recording medium has the shortcomings that the erasing ratio is small and that the laser power required for recording and erasing is not sufficiently reduced.
Furthermore, Japanese Laid-Open Patent Application 1-277338 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by the formula of (Sb.sub.a Te.sub.1-a).sub.1-b M.sub.b, wherein 0.4.ltoreq.a.ltoreq.0.7, b.ltoreq.0.2, and M is one element selected from the group consisting of Ag, Al, As, Au, Bi, Cu, Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn. In this reference, it is asserted that a basic system of the above-mentioned alloy is Sb.sub.2 Te.sub.3, and that the addition of Sb to this alloy in an excessive amount in terms of atomic percentage makes it possible to attain high-speed erasing operation and improve the repetition properties, and that the addition of the element M can further enhance the high-speed erasing performance. In addition to the above advantages, this reference asserts that an erasing ratio obtained by the application of DC light is large. However, this reference does not show any specific erasing ratio at overwriting operation, and according to the experiments conducted by the inventors of the present invention, non-erased portions were observed in the course of the overwriting operation and the recording sensitivity obtained was insufficient for use in practice.
Japanese Laid-Open Patent Application 60-177446 discloses an optical recording medium which comprises a recording layer comprising an alloy with a composition represented by formula of (In.sub.1-x Sb.sub.x).sub.1-y M.sub.y, wherein 0.55.ltoreq..times..ltoreq.0.80, 0.ltoreq.y.ltoreq.0.20, and M is one element selected from the group consisting of Au, Ag, Cu, Pd, Pt, Al, Si, Ge, Ga, Sn, Te, Se and Bi. In addition, Japanese Laid-Open Patent Application 63-228433 discloses a recording layer of an optical recording medium, which comprises an alloy with a composition of GeTe--Sb.sub.2 Te.sub.3 --Sb (excess). The recording sensitivity and erasability required for the recording medium cannot be satisfied by any of the above-mentioned conventional optical recording media.
In addition, Japanese Laid-Open Patent Application 4-163839 discloses an optical recording medium provided with a recording layer comprising a Te--Ge--Sb alloy, with the addition thereto of nitrogen atom; Japanese Laid-Open Patent Application 4-52188 discloses an optical recording medium provided with a recording layer comprising a Te--Ge--Se alloy, with at least one element of Te, Ge or Se being a nitride; and Japanese Laid-Open Patent Application 4-52189 discloses an optical recording medium provided with a recording layer comprising a Te--Ge--Se alloy, with nitrogen atom being adsorbed thereby.
These conventional optical recording media do not have sufficient characteristics for use in practice, in carticular, with respect to the improvement of recording sensitivity and erasing sensitivity, the prevention of the reduction of erasing ratio at overwriting, and the extension of the life of recorded and unrecorded portions.
With the rapid spread of compact discs (CD), write-once compact discs (CD-R) capable of writing data therein only once have been developed and placed on the market. In the case of the write-once compact discs (CD-R), however, once erroneous data is written in the CD-R, the written data cannot be corrected, so that it cannot help discarding the CD-R. Under such circumstances, there has been a demand for a rewritable compact disc that can be used in practice.
As one example of the above-mentioned rewritable compact discs, a magneto-optical recording medium has been developed, but has the shortcomings that overwriting operation is difficult to perform, and that the compatibility with CD-ROM or CD-R is poor. Under such circumstances, researches have been actively made for development of a practically usable phase-change optical recording disc which can ensure, in principle, the compatibility with the CD-ROM or CD-R.
Research and development activities for such a rewritable compact disc, using the phase-change optical recording medium, are reported, for example, in the following references: "Proceedings of the 4th Symposium on Phase-Change Recording" p.70 (1992), Furuya et al.; "Proceedings of the 4th Symposium on Phase-Change Recording" P.76 (1992), Jinno et al.; "Proceedings of the 4th Symposium on Phase-Change Recording" p.82 (1992), Kawanishi et al.; Jpn. J. Appl. Phys. 32 (1993) p.5226, T. Handa et al.; "Proceedings of the 5th Symposium on Phase-Change Recording" p.9 (1993), Yoneda et al.; and "Proceedings of the 5th Symposium on Phase-Change Recording" p.5 (1993), Tominaga et al. However, the overall performance of the rewritable compact discs reported in these references is satisfactory, for instance, in view of the drawbacks with respect to the compatibilities with CD-ROM and CD-R, recording and erasing performance, recording sensitivity, permissible repetition number of overwriting operations, permissible repetition number of reproducing operations, and shelf stability. These drawbacks are mainly ascribed to low erasability caused by the composition and structure of a recording material employed in each compact disc.
Under such present circumstances, there is increasingly demanded for development of a phase-change recording material with high erasability and high recording and erasing sensitivities, and also for development of a phase-change rewritable compact disc with excellent overall performance.
To meet this demand, the inventors of the present invention have discovered and proposed Ag--In--Sb--Te based recording materials, for example, as disclosed in Japanese Laid-Open Patent Applications 4-78031 and 4-123551; Jpn. J. Appl. Phys. 31 (1992) 461, H. Iwasaki et al.; "Proceedings of the 3rd Symposium on Phase-Change Recording" p.102 (1991), Ide et al.; and Jpn. J. Appl. Phys. 32 (1993) 5241, H. Iwasaki et al.
In October of 1996, "Compact Disc Rewritable PART III: CD-RW Version 1.0", which is generally called "Orange Book", was issued as the standards for the rewritable compact disc (CD-RW).
Furthermore, special attention has been paid to a digital video disc (DVD) and a DVD-RAM as the major optical recording media for the coming century. As a matter of course, the aforementioned rewritable compact disc is required to have reproduction compatibility with the DVD. However, recording signals of the conventional CD-RW do not exhibit any sufficient reflectance and modulation degree near a wavelength of 650 nm which corresponds to a reproduction wavelength of the DVD, so that the conventional CD-RW do not have sufficient signal characteristics for use in practice.
The above-mentioned standard book "Compact Disc Rewritable PART III: CD-RW Version 1.0" describes the standards for the rewritable compact disc for 2.times. nominal CD speed (2.4 to 2.8 m/s). However, a long recording time is required at such a low linear speed, so that a demand for a rewritable compact disc capable of performing high speed recording has been increased.
In accordance with the development of the CD-RW, drive systems for the CD-RW have also been actively developed, and matching tests between the CD-RW and the drive systems therefor have been made. The results of such matching tests indicate that in some drive systems, errors increasingly occur at the reproduction at 6.times. nominal CD speed or more (7.2 m/s or more), so that it has been confirmed that such high linear speed reproduction is difficult to perform by such drive systems. Furthermore, it is confirmed that in some drive systems, an optimum recording light power for the CD-RW cannot be determined by an "Optimum Power Control procedure" (hereinafter referred to as OPC procedure) which is defined in the aforementioned "Compact Disc Rewritable PART III: CD-RW Version 1.0".
The phase-change optical recording medium such as the CD-RW is initialized in the course of the manufacturing thereof, with a recording layer of the recording medium being crystallized, using an initializing apparatus. The initialized state of the recording layer has a significantly sensitive effect on the overwriting performance of the recording medium. The initialized state also significantly depends upon the initializing apparatus employed. In order to control the initialized state of each recording medium, it is necessary that the initializing apparatus be traced and identified with each recording medium. However, a system for performing such trace control has not yet been developed.
CD-RW is handled with the hand, so that dirt such as oil and dust may be deposited on the front and back surfaces of the disc. If this takes place, a cloth will be usually used to wipe the surfaces of the disc. are covered with dirt such as oil and dust. However, since a standard substrate of the disc is made of polycarbonate, if the surface of such a polycarbonate substrate is wiped to remove such dirt therefrom with a cloth, the surface of the substrate of the CD-RW is scratched, and there is a risk that recording and reproduction cannot be performed any longer due to the scratches formed on the surface of the substrate. To avoid such a problem, it is proposed to provide a hard coat layer such as an ultraviolet-curing resin layer on a non-grooved surface of the substrate, that is, a mirror surface of the substrate. Such a UV-curing resin layer has already been used in the magneto-optical recording disc. However, the coating of the UV-curing resin needs an extremely delicate coating technique, starting from a predetermined position in an extremely narrow area from a molding mark formed on the substrate by injection molding to an innermost groove edge on the substrate. Such delicate coating technique has not ever been applied to the fabrication of a commercially available phase-change optical recording medium. This is because uneven coating of the hard coat layer, no matter how the unevenness is slight, inevitably causes improper initialization of the recording medium.
Generally, the innermost groove edge is concentrically located at a distance of 22 mm from the center of the disc in the optical recording disc such as CD-RW, while in the magneto-optical recording disc, the innermost groove edge is located at a distance of 25 mm or more from the center thereof, so that the innermost groove edge of the optical recording disc is closer by 2 mm or more to the injection molding mark of the substrate than the innermost groove edge of the magneto-optical recording disc. Therefore, in the case of the phase-change optical recording disc, it is extremely difficult to provide the UV-curing resin layer on the substrate thereof with stable reproducibility.
A phase-change optical disc comprising an Ag--In--Sb--Te based recording layer is conventionally known to exhibit excellent recording performance. However, a phase-change optical recording disc which can securely ensure the compatibility with the CD-R, and satisfy the above-mentioned overall performance required for the rewritable compact disc has not yet been provided.